The present invention relates to the rotary drilling of deep wellbores, particularly high-angle or "extended reach drilled" boreholes, and is particularly concerned with improved tubular drill string members, such as drill collars or drill pipe which incorporates contoured circumferential surfaces aiding in the generation of longitudinal forces towards the bottom of a deviated borehole when the drill string is rotated.
When drilling wellbores with deviated boreholes at angles which reach or exceed 60.degree. from the vertical, the ordinary problems encountered in drilling deviated boreholes are magnified and new problems are generated. Among these new problems which arise is the obtention of an adequate weight-on-bit (WOB) at the bottom of the drill string. Essentially, weight-on-bit can be applied to the bottom of the drill string in various ways other than the utilization of the (passive) force of gravity which entails the weight of the drill string acting on the bit.
One feasible method of applying weight-on-bit consists in the use of drill collars or drill pipe which an external circumferential "spirally grooved" or "threaded" surface configuration adapted to produce a longitudinal force towards the bottom of the drill string upon rotation of the pipe during conveyance of the return flow of drilling fluid or mud over the spiral surface. Presently known drill collars which incorporate this "spiral" outer surface configuration have not been able to meet this problem in a satisfactory manner due to a surface "spiral" contour which is not suitably correlated with the required weight-on-bit conditions.
2. Discussion of the Prior Art
Described in U.S. Pat. No. 3,194,331 to E. P. Arnold is an invention relating to drill collars employed in the rotary drilling of deep wells, with the drill collars being coupled in multiple successions at the lower end of a tubular drill string for controlling weight-on-bit and for cooperation with the wellbore well in minimizing borehole deviation. A specially designed groove configuration is provided on the exterior and circumference of a drill collar. During drilling rotation of the drill string return flow of drilling fluid or mud upwardly through the annulus of the borehole about the drill string will react downwardly on the upwardly facing groove surfaces and produce an auxiliary lead acting downwardly on the bit supplemental to drill collar weighting. This will facilitate the replacement of a number of drill collars with less expensive drill pipe.
Similar structures for drill collars and drill pipes incorporating the same concept as Arnold and intended for substantially the same purpose, may be found in U.S. Pat. Nos. 2,999,552; 3,146,611; 3,360,960 and 3,554,307. All of these patents provide for spirally grooved or threaded circumferential surfaces which will assist in the weighting of the drill bit.
Although structural similarities are in evidence with respect to the tubular drill string member of the present invention and the above-mentioned prior art publications, none of the prior art patents are adapted to engage the wall of the borehole and to propagate there along due to the contoured outer surface of the tubular member so as to generate a longitudinal force towards the bottom of a deviated borehole upon rotation of the drill string.